π-Conjugated Copolymers Composed of Boron Formazanate and Their Application for a Wavelength Converter to Near-Infrared Light

We report conjugated polymers consisting of boron formazanate in the main chains. By employing typical donor-type comonomers, such as fluorene and bithiophene, copolymers were prepared with the Migita–Kosugi–Stille cross-coupling polymerization. Their electronic structures were evaluated by optical and electrochemical measurements. Absorption and emission bands were obtained in the deep-red and near-infrared (NIR) regions. A conjugated polymer composed of formazanate and fluorene, especially, exhibited remarkable NIR emission at 751 nm with 0.14 of absolute quantum yield. Further, boron formazanate works as a strong electron-accepting unit in main-chain conjugation. Therefore, emission from the polymers has the intramolecular charge transfer character. Finally, we demonstrate the application for wavelength-conversion materials from visible to NIR light based on partially modified polymers. By changing complexation ratios in the polymer main chains, broad absorption bands covering the whole visible region and the single NIR emission band were obtained. As a result, the scheme for visible light absorption followed by the NIR emission has been constructed.